[css-shadow-parts-1] Add IDL for part.

Progresses w3c#2414

Author: fergald

css-shadow-parts-1/Overview.bs

@@ -301,4 +301,19 @@ but never match additional <a>shadow-part pseudo-elements</a>.
     ignoring any other labels.
 </div>
 
-Issue(w3c/csswg-drafts#2414): Define IDL for structured setting and getting of `part` and `exportparts`.
+Extensions to the {{Element}} Interface {#idl}
+==============================================
+
+<pre class=idl>
+partial interface Element {
+  [SameObject, PutForwards=value] readonly attribute DOMTokenList part;
+};
+</pre>
+
+The part attribute’s getter must return a DOMTokenList object
+whose associated element is the context object
+and whose associated attribute’s local name is part.
+The token set of this particular DOMTokenList object are also known as the element’s parts.
+
+Issue(w3c/csswg-drafts#2414): Define this as a superglobal in the DOM spec.
+Issue(w3c/csswg-drafts#2414): Define IDL for structured setting and getting of `exportparts`.